Blue Diamond means the body is grounded. (By grounded it is meant that the body is constrained , i.e the body does not have any degree of freedom)
I will attach the meaning of all the symbols soon
A grounded relationship as you are describing is basically the part is locked down, in whatever location it currently is at, and cannot be moved. Usually this is something that was either placed automatically when a part is created in place (if you have the autogrounding checkbox checked when creating in place, or sometimes when an assembly is translated from another format, or also if you drag and drop a part into the assembly while holding down ctrl. When you hold the ctrl key what it is basically doing is matching the coordinate system of the part being placed withthe coordinate system of the assembly, though it places the part as such, but just creates a grounded relationship.
So basically when you see that symbol, it means you can't alter that parts location unless you supress or delete that grounded relationship.